Lacquer without sealer

A beginner asks for opinions on his system and gets the pros and cons of using a sealer. August 29, 2001

I have just started spraying lacquer without using a sealer. I mixed 1 part thinner to 2 parts lacquer and got good results. I sanded to 220 on oak ply, then sprayed 4 coats of the mix, allowing only enough time to dry to touch in between. The gloss lacquer has a semi-gloss appearance (though this may be subjective), and the finish is smooth and even. I would say a success. Since it's my first time with lacquer, should I expect anything to go awry using this procedure?

Forum Responses
From contributor D:
In fact, it's actually better NOT to use a sealer. The sealer is used only because it's easier to sand. Zinc stearate, the material with which the sealer is loaded so that it is easier to sand, softens the film and decreases the water resistance of the lacquer topcoats. It's used as a quicker way to get a smooth surface as it also fills the wood pores. If you don't mind the harder sanding and the quicker loading of your sandpaper, what you're doing is fine and in fact superior. Vinyl sealer, on the other hand, does present some advantages with respect to water resistance, but conventional stearated lacquer sanding sealer is a crutch, not a better way to do finishing.

I use a thin coat of lacquer for a sealer all the time. I don't need two setups to spray. The only word of warning is to use a very thin coat on veneer faced MDF as a sealer. If the first coat is too thick, it will bubble, probably from outgassing of the MDF.

Contributor D was right on with respect to stearated sealer. However, we've found many advantages to using a single coat of non-stearated vinyl sealer. Besides what was mentioned, using a vinyl sealer is faster since the first coat builds nicely and can be sanded more aggressively to give a flatter base for the lacquer. More importantly, the vinyl sealer layer acts as a boundary layer, making it possible to get a more fail-safe lacquer finish on difficult woods, such as rosewood.

Everyone is correct here. Contributor D hit the nail right on the head as far as lacquer versus sanding sealer. The improvement I think you could make would be to reduce the lacquer 50%. This will help promote good adhesion. Also, on your four coats of topcoat, stop after two or three coats, then sand and do a final topcoat of two coats with a little lacquer retarder in the mix. If you ever do use a sanding sealer, you will be spoiled and not want to go back to using lacquer as a sealer.

Someone has to defend sanding sealers, so I will. Personally, I would never design a finish system using lacquer topcoat as my sealer, particularly in a higher production mode.

Sealers are designed differently for a reason. Fast dry and a little harder film so when you sand, the wood fibers break off clean, resulting in a smoother base. Easy sanding is not a negative. They are also built to promote adhesion to wood, stains, etc.

Lacquer topcoats are slower drying and more flexible. Their sanding capabilities are marginal, especially on softer wood, poor sanding--poor quality.

Build coats with topcoat because of their flexibility. Sanding between coats is a plus.

Moisture resistance in pure lacquer is poor no matter how you spray it. Vinyl sealer helps, but in relative terms...not much.

If you want a beautiful furniture finish, use lacquer. If you want higher integrity films, you have to use high integrity coatings.

On the subject of sealer coats, how does shellac do as a base coat for NC lacquer? I use it all the time under WB, but was just about to expand my spray options to include solvent lacquer. I had intended to use de-waxed shellac under that as well.

I will also defend sealers. Talk to any formulating chemist--sealers are sealers and topcoats are topcoats. On all these self-seal systems, they have combined some of the benefits of both in one product, but they will never get a topcoat to do the job of a sealer. I discussed with one company the formulations of the pre-cat sealer and topcoat. Both used the same resin system and almost the same solvent lineup. All the differences are in the additives in the product to make it do what you want.

Laying down too much sealer is not good, but using the right amount can have many benefits. The drive for self-seal systems stems from the mid-sized shop that does not want two products in-house. All large production shops still use sealers along with the finer furniture companies. All these systems have their place. I don't feel that self-seal will ever take over.

Bob Niemeyer, forum technical advisor

From contributor D:
Bob, I disagree. For example, almost all modern conversion varnishes are self-sealing. Akzo Nobel (Rel-Plaz) and Lilly (Resistovar) recommend against using a sealer. True, you need to greatly reduce these high solids coatings for the first coat so that they penetrate and give good adhesion, but the root coating, even though it is reduced, is the same and works great.

Also, the pre-cat that I favor (Unicoat from MacLac - R. J. McGlennon in San Francisco) sands just as easily as most vinyl sealers. It seems to me that sealers are used exclusively to speed production (they dry faster and fill pores because of the stearate) but they do not improve the performance of the coating system. The exception to this is sealers used as barrier coats such as urethane for polyester or vinyl sealer for oil stains. These serve a distinct purpose as bridging films. But old-fashioned stearated lacquer sealer does not.

Everyone is right to different degrees.

With a quality coating line, which nowadays everyone has available to them, sanding sealers are designed to do a job. Vinyl is used to bridge over the grain, helping seal the wood from moisture. Most quality sealers are designed to set up before sinking into the wood, keeping the coating on top of the surface, yet attaching to the wood at the same time. When you get into coatings other then NC, almost all of them make excellent self-sealers. Usually, they need to be thinned down a good deal, but they have good bridging properties and stay on the surface. Adhesion becomes an important factor and CV or pre-cats are much more sensitive, making it very common to use them as a self-sealer. Going back to the original person who was using NC, it sounds like he doesn't have a large volume, so using his NC topcoat isn't a problem. If he had a large volume, he would use less coating by putting a good sealer into his system.

The only time I would put shellac into the system is with pine, where I might want to seal the knots or with water base when using white stains on oak.

Some of the best sealers I've seen are the two part urethanes from Italy. They work great under CV or polyesters. But if you design a topcoat to be a sealer, you are going to have to give up some topcoat qualities and vise versa.

From contributor J:
I wouldn't design a finish system without a sanding sealer, either. Having used both Akzo's Rel-Plaz and Lilly's resistovar (I highly prefer Rel-Plaz over Resistovar) and other conversion varnishes, the comparison of sanding a conversion varnish over a lacquer are two completely different animals. You just can't compare the two for sanding properties.

I agree with what Bob said about the self-sealing systems (which I will never understand because of the resin being the same less stearate and solvent line up changes) being more suited for smaller shops using only one product. I want to add that the sealer needs to be thinned more than the topcoat for good adhesion. You never want to replace excess amounts of sanding sealer for less topcoat and more fill. Sanding sealer, when used correctly, is part of the foundation of a good quality finish. In production, you cannot live without it. Sanding sealer is designed to raise any loose grain, penetrate the wood and sand easily for further coats to have a smooth surface to bond to.

Were you comparing Resistovar or Global Resistovar (the newer product built off of the HAPS compliant Super Chemveer) to Rel-Plaz? Why specifically do you like Rel-Plaz better?

From contributor J:
It was the old Resistovar. Resistovar was also my first experience with a conversion varnish and I don't think it was as widely used as today. Since then, conversion varnishes that I have worked with have been better. Don't get me wrong, Resistovar is a good product and perhaps (with hindsight) if I had run this product through a conventional spray gun in a stainless steel pressure pot rather then doing it air assisted airless, we would have had better success with this product. I haven't ever used the Global Resistovar, just the old formulation, and this was about 10 years ago. Rel-Plaz is one of my favorite conversion varnishes.

The small- to mid-size cabinetmaking firm usually has limited resources in finishing equipment (guns). A self-sealing lacquer can be to the advantage of such an enterprise. Who here has not sprayed sanding or vinyl sealer on 11 of 12 pieces and changed product to topcoat, only to find that one piece had been forgotten? Not only that--consider the inventory implications. And what about human error (now what's in this gun.... sealer or lacquer?)?

Becker Acroma's product line is almost exclusively self-sealing. From their pre-catalysed Intro to their extremely high-solid conversion varnish Euroclear (56%), these products are meant to be used as sealer, as well as topcoat. Vinyl and sanding sealers are usually softer solids. Using a self-sealing product will give you a more protective finish. I have yet to come across a Becker product that is difficult to sand.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
For a long time I did not use sanding sealers. Then one day, after spraying a 12' long double-sided book case for a school, I found the glue. A couple of nice 8-year-old size palm prints from my helper. I spent way too much time stripping and refinishing. Ever since then, I shoot a coat of sanding sealer first, just to be sure. I can't say the results are any different, but the job can be much easer.

Comment from contributor M:
I've been using the pre-cat self sealing lacquer, undiluted, and I use a dull piece of 150 to sand between the 2 coats for quicker sanding. I find no difference in the finish except perhaps the gloss of the finished product (slightly lower gloss), but not enough to make me stop using it. I've used sealer with 320 sand paper, but find I have more problems with dust ending up in the final coat than with what I'm using now. In the end you still have to look at more than the product you use. You have to look at the whole system and keep it in perspective.